This invention describes a reductive amination process whereby perfluorinated ketones or ketals are combined with α-aminoesters under basic conditions to form imine metal carboxylates. Diastereoselective reductions of the imine metal carboxylates enable access to two diastereomers, depending on the reducing conditions. These diastereomers can be further substituted to provide selective cathepsin K inhibitors which can be used in the treatment of osteoporosis and osteoarthritis.
The art involved imines of amino esters, where as the instant invention involves imines of carboxylic acid salts. In addition, the previously reported substrates do not contain fluorinated imine substituents. The selectivities obtained under the conditions described herein are much higher than those presented in the art.
This invention solves two problems. What is needed in the art is a process for the formation of imines from ketones or ketals bearing fluorines in the alpha position. This is a difficult transformation due to the stability of an initially formed hemi aminal intermediate which is difficult to dehydrate to give the imine. This problem had previously been solved by using strongly acidic solutions and high temperatures, which generally leads to decomposition of the starting materials and low yields of the required imine. The process of this invention uses basic conditions for the imine formation and has a number of advantages, including: the imines are formed under much milder conditions (between 20 and 50° C. and the presence of MeO−M+ or M+2CO3=) which gives more efficient recovery (˜90%); a wider range of substrates are tolerated by these conditions; and the imine product is a stable carboxylic acid salt which upon reduction can give a corresponding substituted amino acid product. This avoids a potentially problematic ester hydrolysis step which can lead to epimerization of the amino acid stereocenter. Also, by the careful choice of reducing conditions, the two amino acid diastereoisomers can be accessed from the same imine metal carboxylate.